This invention relates generally to electronic circuits and more particularly to a transmitter which achieves high linearity with low power dissipation.
In many electronics applications, it is necessary to send digital information from one machine to another machine across a transmission medium. In such applications, modems are often used. With a modem, it is possible to convert a set of digital signals into a set of analog signals, and then to send the analog signals across the transmission medium to another modem. It is also possible to receive a set of analog signals from another modem, and then to convert the analog signals back into a set of digital signals which may be processed by a digital mechanism. Thus, using modems, digitally based mechanisms can exchange information across an analog transmission medium.
In a typical modem design, an integrated analog front end (AFE) is used to interface the digitally-based mechanism (e.g. a computer) with the transmission medium (e.g. a twisted pair copper line). The AFE typically comprises two portions: (1) a transmitter; and (2) a receiver. The transmitter portion is responsible for converting digital signals from the digitally-based mechanism into analog signals, and then transmitting the analog signals onto the transmission medium. The receiver portion is responsible for receiving analog signals from other modems via the transmission medium, and then converting them into digital signals that can be processed by the digital mechanism. Together, these two portions provide the communications capabilities for the modem. Typically, the AFE of a modem is implemented as part of an overall integrated circuit.
With reference to FIG. 1, there is shown a typical embodiment for the transmitter portion of an AFE. As shown, the transmitter 100 comprises a digital-to-analog converter (DAC) 102 and a line driver 104. The DAC 102 is responsible for converting a set of digital signals from a digital mechanism (not shown) into a set of analog signals, while the line driver 104 is responsible for driving the analog signals onto the transmission medium 108. In effect, the line driver 104 acts as a buffer between the DAC 102 and the transmission line 108, which is usually a low impedance line (xcx9c100 Ohms). In this configuration, the line driver 104 uses voltage drive to generate the required output signals. To provide direct current (DC) isolation between the line driver 104 and the transmission line 108, the line driver 104 is coupled to the transmission line 108 via a transformer 106. In addition to DAC 102 and line driver 104, the transmitter 100 further comprises a pair of line termination resistors 110, 112. These resistors 110, 112, each having an impedance equal to half the impedance Rline of the transmission line 108, act as termination impedances for incoming analog signals (since incoming signals also go through the transmission line 108).
In order to properly drive analog signals onto the transmission line 108, the transmitter 100 needs to generate output signals which cause signals having the proper peak signal levels to appear on the transmission line 108. These peak signal levels are determined by various communications standards. Because of the presence of the termination resistors 110, 112, the line driver 104 needs to be able to generate voltages which are at least twice that of the peak signal levels. Given sufficiently large power supply voltages, this is not a problem. However, as noted previously, the AFE transmitter 100 is typically implemented as part of an overall integrated circuit, and as fabrication techniques improve, the power supply voltages provided to such integrated circuits have steadily decreased. They have now decreased to the point where the line driver 104 can no longer generate the necessary signal levels with a 1:1 turns ratio in the transformer 106. It is possible to achieve a voltage gain of n by changing the turns ratio of the transformer 106 from 1:1 to 1:n where n is greater than 1. The problem with this approach, however, is that it causes the current drive required of the line driver 104 to increase by the same factor n, which in turn increases the power dissipation of the line driver 104 by a factor n. Thus, the proper signal levels can be achieved, but at the expense of a significant increase in power dissipation. Since it is usually desirable to keep power dissipation to a minimum, this is not an attractive solution.
As an alternative, a more power efficient class AB current drive architecture may be used to limit power dissipation. While this approach is effective from a power standpoint, it is not effective from a performance standpoint. The class AB architecture typically suffers from poor linearity performance due to crossover distortion. Hence, neither approach is wholly satisfactory. As a result, an improved transmitter circuit is needed.
The present invention provides an improved transmitter which is capable of achieving high linearity with minimal power dissipation. According to one embodiment, the present invention comprises a digital phase splitter and an output stage. The digital phase splitter comprises a positive phase DAC for converting the positive phase portion of a set of input digital data into an analog signal, and a negative phase DAC for converting the negative phase portion of the set of input digital data into another analog signal. One of the main functions of the phase splitter is to separate, in the digital domain, the positive and the negative phases of the input digital signal, and to generate corresponding analog signals based upon the two phase portions. Once generated, the analog signals from the positive and negative phase DAC""s are fed as inputs to the output stage. In one embodiment, the digital phase splitter is implemented as part of an overall integrated circuit which is powered by a power supply having a supply voltage of Vcc.
The output stage receives the analog signals from the DAC""s, and in response, drives the analog signals onto a transmission medium, such as a transmission line, by way of a transformer. In doing so, the output stage generates, at its output, signals having the proper signal levels. These output signals are generated by a first and a second current sinks within the output stage, both of which are coupled to the output of the output stage.
In one embodiment, the output stage is implemented separately from the digital phase splitter using discrete components, and is powered by a power supply voltage Vdd which is separate from and is greater than the power supply voltage Vcc used to power the digital phase splitter integrated circuit. Supply voltage Vdd is selected such that it is sufficiently large to enable the output stage to generate the necessary output signal levels with just a 1:1 turns ratio in the transformer. By separating the output stage from the digital phase splitter integrated circuit, and by operating the output stage at a higher supply voltage, the present invention eliminates the need to increase the turns ratio of the transformer, and hence, eliminates the need to increase the current drive and power dissipation of the output stage. By doing so, the present invention makes it possible to drive the necessary signals onto the transmission line without increased power dissipation.
To further reduce power dissipation, the output stage is operated in a power efficient class AB current drive mode. In this mode, the current sinks are not turned fully off when they are not generating signals but instead are maintained in a semi-on, xe2x80x9cidlexe2x80x9d state by a small bias voltage. Operating the output stage in this mode helps to reduce power dissipation. Normally, class AB current drive mode is not a viable option in applications of this type because of the poor linearity performance caused by crossover distortion. However, because the present invention splits the phases of the input digital signal in the digital domain, and then generates corresponding analog signals based upon the split phase signals prior to the output stage, the output stage experiences minimal crossover distortion. Hence, the output stage is able to achieve high linearity even when operated in class AB current drive mode. In this manner, the present invention is able to minimize power dissipation without suffering from poor linearity performance.